With the recent development of the electronic and electric industries, simplification, size reduction and high reliability have been demanded in mounting circuit elements in appliances and communication equipment, and the importance of printed circuit boards has been increasing. Of various printed circuit boards, those using a flexible base are advantageous for size and weight reductions and capable of surface mounting.
Flexible bases are generally produced by laminating a copper foil on a heat resistant film, such as a polyimide film, with an adhesive (adhesive lamination) or without an adhesive (two-ply lamination). Where a copper foil is laminated via an adhesive, the heat resistance of the base is largely dependent on the heat resistance of the adhesive, and the heat resistance inherent to the heat resistant film cannot be taken full advantage of. Accordingly, use of a two-ply base obtained by laminating a copper foil on a heat resistant film without using an adhesive has been extending.
On the other hand, a flexible printed circuit board is usually covered with a cover-lay film, such as a polyimide film, in order to protect a printed circuit from moisture or foreign matters. Adhesive lamination of the cover-lay film would also give rise to a problem of reduction in heat resistance. Since most polyimide films have no adhesion by themselves, it has been required to apply an adhesive on the film.
However, epoxy-based adhesives which are widely used in the art do not have sufficient heat resistance. Adhesion structures using a thermoplastic polyimide resin as an adhesive have recently been proposed. While excellent in heat resistance and storage stability, thermoplastic polyimide resins require relatively high temperatures for adhesion and are still unsatisfactory in workability.
Additionally, thermoplastic polyimide resins have a high coefficient of linear expansion. When used for adhesive lamination of a copper foil or a cover-lay film, they sometimes fail to obtain sufficient dimensional stability or cause curling due to the difference from an adherend in linear expansion coefficient.
It has thus been demanded to develop a cover-lay film, a laminate film (such as a two-ply base) or an adhesive film having excellent workability, dimensional stability, and heat resistance in which or with which adhesion is achieved with no adhesive.